SIGNALING RESPONSES TO ALKYLLYSOPHOSPHATIDIC ACID - THE ACTIVATION OFPHOSPHOLIPASE-A(2) AND PHOSPHOLIPASE-C AND PROTEIN-TYROSINE PHOSPHORYLATION IN HUMAN PLATELETS

The profile of biochemical responses following stimulation of human pl atelets with 1-alkyl-2-lyso-sn-glycero-3-phosphate (ALPA), a derivativ e of platelet-activating factor (PAF), was investigated, In the presen ce of extracellular Ca2+, ALPA evoked a dose-dependent increase and su stained elevation of the intracellular free Ca2+ concentration and sti mulated the formation of phosphatidic acid, Platelets released free [H -3]arachidonic and [H-3]oleic acid at maximal rates between 5 and 15 s following ALPA stimulation, However, in platelets labeled with myo-[( 3H)]inositol, ALPA induced [H-3]phosphoinositide breakdown and formati on of [H-3]inositol phosphates with slower kinetics, Intracellular Ca2 + mobilization and the release of free fatty acids and inositol phosph ates were not inhibited by pretreatment of platelets with pertussis to xin (PTX) or the PAF receptor antagonist WEB 2086. Following platelet stimulation with ALPA, tyrosine phosphorylation of proteins with appar ent molecular masses of 65-95, 110-130, and 145-170 kDa was increased in a time-dependent manner, while phosphorylation of 40- to 45-kDa pro teins was decreased, One of the platelet proteins phosphorylated on ty rosine residues in response to ALPA was found to be PLC-gamma 1, Exoge nous [H-3]ALPA was metabolized primarily to [1-H-3] alkyl-2,3-diacyl-g lycerol, The metabolic conversion of [H-3]ALPA involved a dephosphoryl ation reaction, and the formation of the dephosphorylated product, [1- H-3]alkyl-monoglycerol, was detected within 5 s, These data demonstrat e that an ether-linked lysophosphatidic acid can activate human platel ets by a PTX-insensitive mechanism which does not involve the PAF rece ptor. Upon stimulation of platelets, ALPA induces the activation of ph ospholipases A(2) and C, and tyrosine phosphorylation of several cellu lar proteins including PLC-gamma 1. These signal transduction response s in platelets are accompanied by rapid metabolism of ALPA. (C) 1996 A cademic Press, Inc.